Intervertebral disks often become damaged creating discomfort for a patient. It is known that partial or total removal of a damaged intervertebral disk may alleviate some of this discomfort but may limit the natural function of the spine. Thus, nowadays following the removal of a damaged, natural intervertebral disk or a damaged nucleus pulposus of a natural intervertebral disk, an implant or prosthesis is placed in the intervertebral space between the two vertebra adjacent to the site of removal. The aim of implanting such devices is to bring about an as natural state as possible, particularly duplicating the original height of the intervertebral disk and consequently restoring the original distance between the two adjacent bodies of the vertebra. Furthermore, movements of adjacent bodies of the vertebra relative to one another should be able to be carried out with minimal hindrance of their natural function. For this purpose the retention of the ability to move when bending forward or backward, i.e. the flexion and the extension of the bodies of the vertebra as well as the lateral bending of the intervertebral bodies within the natural limits, is desirable. It is also desirable that the natural ligaments and muscles along the spine are left substantially intact, to further stabilize the movements of a mechanical replacement for a intervertebral disk.
Such an intervertebral implant is known from U.S. Pat. No. 5,556,431 to Büttner. This known implant comprises a bottom base plate and a top cover plate, the exterior surfaces of which can be placed on the adjacent bodies of the vertebra, as well as a joint provided between the cover plates. This joint consists of a hemispherical first joint part and a matching joint shell as the second joint part, so that the cover plates can pivot polyaxially relative to one another. A disadvantage of this known intervertebral implant is that although the two cover plates are connected with a joint part each, the joint parts are not held together. This requires the implant to be assembled by the surgeon creating a possibility for erroneous assembly. In-situ the joint parts and consequently the entire implant must be held together using a special instrument. This creates a possibility that the implant will fall apart prior to and during implantation, risking damage to surrounding tissue and risking loss of parts.
From U.S. Pat. No. 5,895,428 to Berry an intervertebral implant is known, that comprises joining means to hold the joint parts together. A disadvantage however, of this known intervertebral implant is that during a relative movement of the two joint parts the joining means slide on one another without any clearance and no lubricating film of the body's own fluids can be formed on them. A lubricating film of the body's own fluids may advantageously reduce wear and erosion between the sliding surfaces.